Current interrupter for electrochemical cells

ABSTRACT

A current interrupt mechanism for electrochemical cells is disclosed. A thermally activated current interrupt mechanism is integrated into an end cap assembly for an electrochemical cell. The thermally responsive mechanism preferably includes a free floating bimetallic disk or shape memory alloy member which deforms when exposed to elevated temperature causing a break in an electrical pathway within the end cap assembly. This prevents current from flowing through the cell and effectively shuts down an operating cell. The end cap assembly may include a pressure responsive mechanism which ruptures when there is extreme gas pressure buildup. Gas is allowed to escape from the cell interior to the external environment through a series of vent apertures within the end cap assembly.

This application is a continuation in part of application Ser. No.08/720,616 filed Oct. 2, 1996 now U.S. Pat. No. 5,691,073 which claimedbenefit of provisional application Ser. No. 60/015,153 filed Apr. 10,1996.

FIELD OF THE INVENTION

This invention relates to a thermally responsive current interrupter foran electrochemical cell, which safely prevents current flow through thecell upon an excessive increase in the temperature thereof. Theinvention also relates to a pressure responsive current interrupter fora cell, which safely shuts down the cell upon excessive gas pressurebuildup therein.

BACKGROUND OF THE INVENTION

Electrochemical cells, especially high energy density cells such asthose in which lithium is an active material, are subject to leakage orrupture which, in turn, can cause damage to the device which is poweredby the cell or to the surrounding environment. In the case ofrechargeable cells, the rise in internal temperature of the cell canresult from overcharging. Undesirable temperature increases are oftenaccompanied by a corresponding increase in internal gas pressure. Thisis likely to occur in the event of an external short circuit condition.It is desirable that safety devices accompany the cell without undulyincreasing the cost, size or mass of the cell.

Such cells, particularly rechargeable cells utilizing lithium as anactive material, are subject to leakage or rupture caused by a rise ininternal temperature of the cell which often is accompanied by acorresponding increase in pressure. This is likely to be caused byabusive conditions, such as overcharging or by a short circuitcondition. It is also important that these cells be hermetically sealedto prevent the egress of electrolyte solvent and the ingress of moisturefrom the exterior environment.

As set forth above, as such a cell is charged, self-heating occurs.Charging at too rapid a rate or overcharging can lead to an increase inthe temperature. When the temperature exceeds a certain point, whichvaries depending upon the chemistry and structure of the cell, anundesirable and uncontrollable thermal runaway condition begins. Inaddition, because of the overheating, internal pressure builds up, andelectrolyte may suddenly be expelled from the cell. It is preferable toinitiate controlled venting before that takes place.

Conventional cell designs employ an end cap fitting which is insertedinto an open ended cylindrical casing after the cell anode and cathodeactive material and appropriate separator material and electrolyte havebeen inserted into the cylindrical case. The end cap is in electricalcontact with one of the anode or cathode material and the exposedportion of the end cap forms one of the cell terminals. A portion of thecell casing forms the other terminal.

SUMMARY OF THE INVENTION

The present invention has one or several current interrupt mechanismsintegrated within a single end cap assembly which may be appliedadvantageously to primary or secondary (rechargeable) cells, forexample, by inserting the end cap assembly into the open end of a casingfor the cell. The end cap assembly of the invention has particularapplication to rechargeable cells, for example lithium-ion, nickel metalhydride, nickel cadmium or other rechargeable cells, to overcome thedanger of the cell overheating and pressure building up in the cellduring exposure to high temperatures, excessive or improper charging, orshorting of the cell.

In one aspect the invention is directed to an end cap assembly for anelectrochemical cell wherein the end cap assembly has integrated thereina thermally responsive current interrupt mechanism which activates tointerrupt and prevent current from flowing through the cell when thecell interior overheats to exceed a predetermined temperature. The endcap assembly has an exposed end cap plate which functions as a terminalof the cell. When the assembly is applied to a cell and the cell is innormal operation the end cap plate is in electrical communication with acell electrode (anode or cathode). The thermally activated currentinterrupt mechanism integrated within the end cap assembly may comprisea bimetallic member that deflects when exposed to temperature above apredetermined value. The deflection of the bimetallic member pushesagainst a movable metal member to sever electrical connection between anelectrode of the cell and the end cap terminal plate thus preventingcurrent from flowing through the cell. Alternatively, in another aspectof the invention a thermally responsive pellet may be used instead ofthe bimetallic member. If the temperature of the cell exceeds apredetermined value, the thermal pellet melts causing a metallic membersupported thereon to deflect sufficiently to sever the electricalpathway between an electrode of the cell and the end cap terminal plate.In another aspect of the invention the thermally responsive currentinterrupt mechanism may include a shape memory alloy member integrateinto the end cap assembly. During normal operation of the cell the shapememory member provides a portion of the electrical pathway between theend cap plate and one of the cell electrodes to allow current to passthrough the cell. When the cell temperature reaches a predeterminedvalue the shape memory member deflects thereby breaking the electricalpathway and immediately interrupting current flow through the cell. Arupturable plate or membrane may be integrated into the end cap assemblyalong with the thermally responsive current interrupt mechanism. Whenpressure within the cell builds up to exceed a predetermined value theplate or membrane ruptures allowing gas from the interior of the cell toescape to the external environment.

In another aspect the invention is directed to an end cap assembly forcells, particularly rechargeable cells, wherein the end cap hasintegrated therein two current interrupt mechanisms one being thermallyresponsive and the other being pressure responsive. The thermallyresponsive current interrupt mechanism may preferably employ abimetallic member, a shape memory member or thermally responsivemeltable pellet which activates to interrupt and prevent current flowthrough the cell when the cell interior overheats to exceed apredetermined temperature. The pressure responsive current interruptmechanism activates to interrupt current flow when gas pressure in thecell builds up to exceed a predetermined value. In such case, thepressure interrupt mechanism may cause a metal diaphragm within the endcap assembly to deflect thereby severing the electrical connectionbetween the cell end cap terminal plate and a cell electrode, therebypreventing current from flowing through the cell. In the case of extremegas pressure buildup the metal diaphragm also ruptures allowing gas tobe channeled into interior chambers within the end cap assembly and outto the external environment through a series of vent holes.

Another aspect of the invention is directed to a sealing mechanism forthe end cap assembly of the invention. The sealing mechanism preventsleakage of electrolyte, liquid or gas from the end cap interior to theexternal environment and prevents ingress of moisture into the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will be better appreciated with referenceto the drawings in which:

FIGS. 1, 2 and 3 are vertical cross-sectional views taken through sightlines 1--1 of the end cap assembly of FIG. 6.

FIG. 1 shows the thermally activated current interrupt mechanism andpressure activated current interrupt mechanism in circuit connectedmode.

FIG. 2 shows the thermally activated current interrupt mechanism incircuit interrupted mode.

FIG. 3 shows the pressure activated current interrupt mechanism inpressure activated, circuit interrupted mode.

FIG. 4 is a vertical cross-sectional view of another embodiment of anend cap assembly having pressure activated current interrupt mechanismand thermally activated current interrupt mechanism integrated thereinin which a heat sensitive member softens to release a resilient memberto open the circuit.

FIG. 5 is an exploded perspective view of the components of end capassembly of the invention shown in the embodiment of FIG. 1 or FIG. 9.

FIG. 6 is a perspective view of the bottom of the end cap assemblyshowing the pressure resistant plate and vent apertures therethrough.

FIG. 7 is a perspective view showing the end cap assembly of theinvention being inserted into the open end of a cylindrical casing of acell.

FIG. 8 is a perspective view showing a completed cell with the end capassembly of the invention inserted into the open end of a cylindricalcasing of a cell with end cap plate of said assembly forming a terminalof the cell.

FIG. 9 is a vertical cross sectional view which shows the thermallyactivated current interrupt mechanism employing a shape memory memberand pressure activated current interrupt mechanism in circuit connectedmode.

FIG. 10 is a vertical cross section view which shows the thermallyactivated current interrupt mechanism employing the shape memory memberin circuit interrupted mode.

DETAILED DESCRIPTION

The end cap assembly 10 (FIG. 1) of the invention may be applied toprimary or secondary (rechargeable) cells. In a preferred embodiment theend cap assembly 10 is insertable into the open end 95 of a typicallycylindrical casing 90 for the cell (FIG. 7). The cells contain apositive electrode (cathode on discharge), a negative electrode (anodeon discharge), separator and electrolyte and positive and negativeexternal terminals in electrical communication with the positive andnegative electrodes, respectively.

Referring now to FIG. 1 of the drawings, an end cap assembly 10 intendedfor insertion into the open end of a cell case comprises a thermallyactivatable current interrupter subassembly 38 and a pressure reliefsubassembly 48 integrated therein. Subassemblies 38 and 48 are separatedby a common support plate 60. Subassemblies 38 and 48 are held within acover 30 which defines the outer wall of the end cap assembly 10.Interrupter subassembly 38 is defined at its top end by a cup-shaped endcap plate 20 and at its bottom end by a contact plate 15 which is weldedto support plate 60. Cup shaped end cap plate 20 forms one of theexternal terminals of the cell. Support plate 60 separates chamber 68within thermal subassembly 38 from chamber 78 within pressure reliefsubassembly 48. Contact plate 15 is electrically connected to supportplate 60 which in turn is electrically connected to an electrode 88(anode or cathode) of the cell when end cap assembly 10 is applied to acell. A thermally responsive circuit interrupter mechanism (40,50) isprovided to complete the circuit between contact plate 15 and end cap20. If temperature within the cell exceeds a predetermined thresholdvalue the interrupter mechanism activates breaking electrical contactbetween end cap 20 and contact plate 15 thereby preventing current fromflowing through the cell.

The pressure relief subassembly 48 comprises a thin metallic diaphragm70 connected to a pressure resistant plate 80 which in turn iselectrically connected to a cell electrode 88 through conductive tab 87which is welded to plate 80. (Pressure resistant plate is electricallyconductive and of sufficient thickness that it does not substantiallydeform at elevated pressures at least up to about 600 psi (4.14×10⁶pascal.) If gas pressure within the cell builds up to exceed apredetermined threshold value diaphragm 70 bulges outwardly to breakelectrical contact with pressure resistant plate 80 thereby preventingcurrent from flowing to or from the cell. Pressure resistant plate 80and support plate 60 preferably also have perforations, 73 and 63,respectively, therein which helps to vent gas and relieve pressurebuildup within the cell.

In the preferred embodiment shown in FIG. 1 end cap assembly 10 may beused in a rechargeable cell, for example, a lithium-ion rechargeablecell. (A lithium-ion rechargeable cell is characterized by the transferof lithium ions from the negative electrode to the positive electrodeupon cell discharge and from the positive electrode to the negativeelectrode upon cell charging. It may typically have a positive electrodeof lithium cobalt oxide (Li_(x) CoO₂) or lithium manganese oxide ofspinel crystalline structure (Li_(x) Mn₂ O₄) and a carbon negativeelectrode. The negative electrode constitutes the anode of the cellduring discharge and the cathode during charging and the positiveelectrode constitutes the cathode of the cell during discharge and theanode during charging. The electrolyte for such cells may comprise alithium salt dissolved in a mixture of non-aqueous solvents. The saltmay be LiPF₆ and the solvents may advantageously include dimethylcarbonate (DMC), ethylene carbonate (EC), propylene carbonate (PC) andmixtures thereof. The present invention is applicable as well to otherrechargeable cells, for example, nickel metal hydride cells and nickelcadmium cells. End cap assembly 10 comprises an end cap terminal 20which is typically the positive terminal of the rechargeable cell, ametal support plate 60 which forms a support base under the cap plate20, and an insulator disk 35 between end cap 20 and support plate 60.Cap assembly 10 is advantageously also provided with a pressure reliefdiaphragm 70 below support plate 60 as shown in FIG. 1. Diaphragm 70 maybe welded to an underlying pressure resistant plate 80. This may beconveniently done by welding the base 72 of diaphragm 70 to a raisedportion 82 of underlying pressure resistant plate 80. Diaphragm 70should be of material that is electrically conductive and of minimalthickness of between about 0.1 and 0.5 millimeter, depending on thepressure at which the diaphragm is intended to actuate. The diaphragm 70may desirably be of aluminum. The diaphragm 70 is advantageously coinedso that it ruptures at a predetermined pressure. That is, the diaphragmsurface may be stamped or etched so that a portion of the surface is ofsmaller thickness than the remainder. One preferred diaphragm 70 for usein the present invention is coined to impose a semicircular or "C"shaped groove 70a in its surface. The shape of the groove advantageouslyis the same or similar to the shape of a major portion of the peripheraledge of diaphragm 70 and positioned advantageously in proximity to theperipheral edge. The particular pressure at which venting takes place iscontrollable by varying parameters such as the depth, location or shapeof the groove as well as hardness of the material. When pressure becomesexcessive the diaphragm will rupture along the groove line. End cap 20and support plate 60 define a chamber 68 therebetween in which issituated a thermally activated current interrupter subassembly 38.Insulator disk 35 is formed of a peripheral base portion 35a and adownwardly sloping arm 35b extending therefrom. Arm 35b extends intochamber 68. Diaphragm 70 is designed to rupture when gas buildup withinthe cell reaches a predetermined threshold level. The region betweensupport plate 60 and diaphragm 70 forms a chamber 78 into which gasbuildup within the cell may vent upon rupture of diaphragm 70.

Current interrupter subassembly 38 comprises a thermally responsivebimetallic disk 40, a metallic contact plate 15 in electrical contactwith a resilient springlike member 50. As shown in FIGS. 1 and 5resilient member 50 may be formed of a single flexible member having anouter circular peripheral portion 50a from which a disk retainer tabportion 50c extends radially inward to generally hold bimetallic disk 40freely in place during any orientation of the cell while not restrictingits snap acting movement. This member can be welded at one point ofouter portion 50a to end cap plate 20 with a center contact portion 50bin contact with plate 15. Additionally, contact portion 50b can bedesigned with a reduced cross sectional area so that it can act as adisintegratable fuse link to protect against power surge conditions.Bimetallic disk 40 is positioned to freely engage sloping arms 35b ofinsulator disk 35 which arms act as the disk seat for disk 40.Bimetallic disk 40 also preferably includes a central aperture forreceiveing a raised contacting portion of metallic contact plate 15.Contact plate 15 is preferably welded to support plate 60 and provides asurface for resilient member 50 to rest as shown in FIG. 1. There is anelectrically insulating grommet 25 which extends over the peripheraledge of end cap 20 and along the bottom peripheral edge of diaphragm 70.Grommet 25 also abuts the outer edge of subassembly 38 as shown inFIG. 1. There may be a ring of metal 55 which is crimped over the topedge of grommet 25 and pressed against diaphragm 70 to seal the end capassembly interior components. The grommet 25 serves to electricallyinsulate the end cap 20 from the crimp ring 55 and also to form a sealbetween support plate 60 and crimp ring 55. The cover 30 of the end capassembly 10 may be formed from truncated cylindrical member shown bestin FIG. 5. In a completed cell assembly (FIG. 8) the outside surface ofcover 30 will come into contact with the inside surface of cell casing90. Support plate 60 provides a base for components of subassembly 38 torest and preferably is of bow shape to maintain active radialcompressive force against the inside surface of grommet 25. Supportplate 60 may be provided with perforations 63 in its surface to vent gasto upper chamber 68 when diaphragm 70 ruptures. Gas which passes intoupper chamber 68 will vent to the external environment through primaryvent holes 67 in end cap 20. The end cap assembly cover 30 is in contactwith the cell casing 90 which is in electrical contact with the oppositeterminal, typically the negative terminal in the case of a lithium-ionrechargeable cell. Thus, grommet 25 provides electrical insulationbetween end cap 20 and outer wall 30, that is, between the two terminalsof the cell thereby preventing shorting of the cell. There may be anadditional insulator ring, namely insulator standoff ring 42 between thetop portion of outer wall 30 and pressure plate 80 as illustrated inFIG. 1, also to assure that there is no shorting between the positiveand negative terminals of the cell.

Diaphragm 70 is preferably in the shape of a cup comprised of aluminumhaving a thickness advantageously of between about 3 and 10 mils. Atsuch thickness the weld between diaphragm base 72 and support plate 80breaks and the diaphragm base 72 bulges and separates from support plate80 (FIG. 3) when internal gas pressure within the cell rises to athreshold value of at least between about 100 psi and 200 psi (6.894×10⁵and 13.89×10⁵ pascal). (Such pressure buildup could occur for example ifthe cell were being charged at higher than recommended voltage or if thecell were shorted or misused.) However, if desired the thickness ofdiaphragm base 72 can be conveniently adjusted to bulge at otherpressure levels. The separation of diaphragm base 72 from plate 80breaks all electrical contact between the diaphragm 70 and plate 80.This separation also breaks the electrical pathway between end cap 20and the cell electrode 88 in contact with plate 80 so that current canno longer flow to or from the cell, in effect shutting down the cell.Even after the current path is broken if the pressure within the cellcontinues to rise for other reasons, for example, heating in an oven,the vent diaphragm 70 will also rupture preferably at a thresholdpressure of at least between about 250 and 400 psi (17.2×10⁵ and27.6×10⁵ pascal) to prevent cell explosion. In such extremecircumstances the rupture of vent diaphragm 70 allows gas from the cellinterior to vent through vent holes 73 (FIGS. 1 and 6) in pressureresistant plate 80 whereupon the gas enters lower chamber 78 (FIG. 1).The gas will then pass from lower chamber 78 to upper chamber 68 throughvent holes 63 in the support plate 60 (FIG. 1) and if needed vent holes(not shown) in insulator disk 35. Gas collected in the upper chamber 68will vent to the external environment through primary vent holes 67 inthe end cap plate 20.

The current interrupt features of the invention may be described withreference to FIGS. 1-3. It should be noted that in the specificembodiment shown therein one of the cell electrodes comes into contactwith plate 80 through tab 87 when the end cap assembly 10 is applied toa cell. During normal cell operation plate 80 in turn is electricallyconnected to end cap plate 20. In a lithium-ion cell the electrode 88 incontact with plate 80 may conveniently be the positive electrode. Thiselectrode will be insulated from the cell casing 90. The negativeelectrode (not shown) will be connected to the cell casing 90. Theembodiment of FIG. 1 shows the end cap assembly configuration beforecurrent is interrupted by either activation of the thermal currentinterrupter bimetallic disk 40 or activation of pressure reliefdiaphragm 70. In the specific embodiment illustrated in FIG. 1 plate 80is in electrical contact with diaphragm 70 and diaphragm 70 is inelectrical contact with support plate 60. Support plate 60 is inelectrical contact with contact plate 15 which is in electrical contactwith resilient member 50 which in turn is in electrical contact with endcap 20. In the integrated end cap design of the invention shown in FIG.1, electrical contact between the electrode 88 in contact with pressureplate 80 and end cap 20 may be interrupted in two ways. As abovedescribed if pressure builds up in the cell to a predeterminedthreshold, contact between diaphragm 70 and pressure plate 80 is brokenas diaphragm base 72 bulges away from pressure plate 80. Thisinterruption in the circuit prevents current from flowing to or from thecell. Alternatively, if the cell overheats the bimetallic disk 40 ofthermal interrupt subassembly 38 activates and in so doing pushesupwardly from insulator 35b thereby causing resilient member 50 todisengage from contact plate 15. This in effect severs the electricalpath between electrode tab 87 and end cap 20, thus preventing currentfrom flowing to or from the cell. It is an advantage of the invention toincorporate these two interrupt mechanisms within a single end capassembly 10 which is insertable into the open end of a cell case as asingle unit.

The bimetallic disk 40 is preferably not physically attached tounderlying insulator disk 35 but rather is free to move, that is itrests in free floating condition on disk arm 35b as shown in FIG. 1. Insuch design current does not pass through the bimetallic disk 40 at anytime regardless of whether the cell is charging or discharging. This isbecause disk 40 when inactivated is not in electrical contact withcontact plate 15. However, should the cell overheat beyond apredetermined threshold temperature, bimetallic disk 40 is designed tothe appropriate calibration such that it snaps or deforms (FIG. 2)causing it to push resilient member 50 away from contact plate 15thereby preventing current from flowing between the cell terminals. Thebimetallic disk 40 is calibrated so that it has a predetermined dishedshape which allows the disk to actuate when a given thresholdtemperature is reached. The free floating design of bimetallic disk 40on insulator disk arm 35b as above described does not permit current topass therethrough at any time regardless of whether the cell is chargingor discharging. This makes the calibration of disk 40 easier and moreaccurate, since there is no heating effect caused by current flowthrough bimetallic disk 40 (I² R heating). Bimetallic disk 40 mayconveniently comprise two layers of dissimilar metals having differentcoefficient of thermal expansion. The top layer of bimetallic disk 40(the layer closest to end cap 20) may be composed of a high thermalexpansion metal, preferably nickel-chromium-iron alloy and theunderlying or bottom layer may be composed of a low thermal expansionmetal, preferably nickel-iron alloy. In such embodiment disk 40 mayactivate when temperature rises to at least between about 60° to 75° C.causing disk 40 to deform sufficiently to push resilient member 50 awayfrom contact with contact plate 15. It is also possible to choose thehigh and low thermal expansion metal layers such that the disk 40 willnot reset except at a temperature below -20° C. which in mostapplications makes the device a single action thermostatic device.

Preferred materials for the above described components are described asfollows: End cap 20 is preferably of stainless steel or nickel platedsteel of between about 8 to 15 mil (0.2 and 0.375 mm) thickness toprovide adequate support, strength and corrosion resistance. The outerwall 30 of the end cap assembly 10 is also preferably of stainless steelor nickel plated steel having a thickness of between about 8 and 15 mil(0.2 and 0.375 mm. Pressure plate 80 is preferably of aluminum having athickness between about 10 and 20 mils (0.25 and 0.5 mm) which may bereduced at the center to between about 2 and 5 mils (0.05 and 0.125 mm)at the point of welded contact with diaphragm base 72. Insulatorstandoff ring 42 may be composed of a high temperature thermoplasticmaterial such as high temperature polyester for strength and durabilityavailable under the trade designation VALOX from General ElectricPlastics Company. Crimp ring 55 is preferably of stainless steel ornickel plated steel having a thickness between about 8 and 15 mils (0.2and 0.375 mm) for strength and corrosion resistance. Diaphragm 70 ispreferably of aluminum having a thickness of between about 3 and 10 mils(0.075 and 0.25 mm). At such thickness the diaphragm will break awayfrom its weld to pressure plate 80 when the internal gas pressureexceeds a threshold pressure between about 100 and 250 psi (6.89×10⁵ and17.2×10⁵ pascal) . Should the internal gas pressure exceed a pressurebetween about 250 and 400 psi (17.2×10⁵ and 27.6×10⁵ pascal) diaphragm70 will rupture to provide additional relief from gas pressure buildup.The insulator disk 35 on which bimetallic disk 40 rests is preferably ofa material of high compressive strength and high thermal stability andlow mold shrinkage. A suitable material for disk 35 is a liquid crystalpolymer or the like of thickness between about 10 and 30 mils (0.25 and0.75 mm) available under the trade designation VECTRA from the CelaneseCo. Support plate 60 is preferably of stainless steel or nickel platedsteel to provide adequate strength and corrosion resistance at athickness of between about 10 and 30 mils (0.25 and 0.75 mm). Resilientmember 50 is advantageously formed of berylium-copper, nickel-copperalloy, stainless steel or the like which has good spring action andexcellent electrical conductivity. A suitable thickness for resilientmember 50 when formed of beryllium-copper or nickel-copper alloy isbetween about 3 and 8 mils (0.075 and 0.2 mm) to give sufficientstrength and current carrying capability. This material may be plated orinlayed with silver or gold at the contact region to provide lowerelectrical resistance in this area. Contact plate 15 is advantageouslyformed of cold rolled steel plated with a precious metal such as gold orsilver to lower contact resistance and improve reliability. It may alsobe formed of a nickel-copper clad alloy, stainless steel, ornickel-plated steel. Grommet 25 typically is made of polymeric materialsuch as nylon or polypropylene. The seal around the end cap assemblycomponents should be hermetic in order that electrolyte, both in theform of liquid and vapor, is prevented from entering into the end capchambers or from leaving the cell.

After the end cap assembly 10 is completed it may be inserted into theopen end 95 of a cylindrical cell case 90 shown in FIG. 7. Thecircumferential edge of cell casing 90 at the open end thereof is weldedto the outer wall of cover 30 of end cap assembly 10 to provide ahermetically tight seal between end cap assembly 10 and the cell casing90. The radial pressure of the circumferential wall of crimp ring 55against grommet 25 and diaphragm 70 produces a hermetically tight sealaround the interior components of end cap assembly 10.

An alternative embodiment of the end cap design having both a pressurerelief mechanism and thermally activated current interrupt mechanismintegrated therein is shown as end cap assembly 110 in FIG. 4. Theembodiment of FIG. 4 is similar to that described above with respect toFIGS. 1-3 except that a bimetallic disk is not employed to activate thespringlike mechanism. Instead a thermal pellet 175 is provided to hold aresilient springlike member 150 in electrical contact with contact plate115. Contact plate 115 in turn is in electrical contact with end capplate 20. Resilient member 150 may comprise an elongated metallic arm150a which is welded at one end to support plate 60. Support plate 60 isin electrical contact with diaphragm 70 which in turn is welded to araised portion 82 of underlying pressure resistant plate 80. Anelectrode tab 87 is in electrical contact with plate 80. Resilientmember 150 preferably terminates at its opposite end in a cup or convexshaped portion 150b which contacts contact plate 115. There is anelectrical insulator disk 120 over the peripheral edge 60a of supportplate 60 to prevent direct contact between support plate 60 and contactplate 115. Thus, there will be electrical contact between support plate60 and end cap 20 as long as resilient member 150 is held pressedagainst contact plate 115. Support plate 60 in turn is in electricalcontact with aluminum diaphragm 70 which is in contact with plate 80 anda cell electrode 88 through tab 87 when the end cap assembly 110 isapplied to a cell. (End cap assembly 110 may be applied to a cell byinserting it into the open end of a cylindrical casing 90 in the samemanner as above described with reference to the embodiment shown in FIG.1.) Therefore, as resilient member 150 is held pressed against contactplate 115 by thermal pellet 175, there is electrical contact between acell electrode 88 (through tab 87) and end cap plate 20 permittingnormal cell operation. If the cell overheats beyond a predeterminedthreshold temperature pellet 175 melts thereby removing support forresilient member 150. Melting of pellet 175 causes resilient member 150to snap downwardly and break electrical contact with contact plate 115.This in effect severs the electrical pathway between the electrode tab87 and end cap 20 thus preventing current from flowing to or from thecell. If the internal gas pressure within the cell exceeds apredetermined value diaphragm 70 will rupture thereby severingelectrical contact between plate 80 and diaphragm 70 and also allows gasto escape to the external environment through vent holes 63 and 67 insupport plate 60 and end cap 20, respectively.

Preferred materials for the end cap 20, support plate 60, contact plate115 and aluminum diaphragm 70 referenced in the embodiment shown in FIG.4 may be the same as described for the corresponding elements having thesame reference numerals shown in FIGS. 1-3. Contact plate 115 ispreferably formed of stainless steel or nickel plated cold rolled steelplated with silver or gold to lower its contact resistance. Theinsulator disk 120 shown in FIG. 4 is preferably made of a hightemperature thermoplastic material having excellent dielectricproperties. A preferred material for disk 120 may a polyimide availableunder the trade designation KAPTON from E. I. DuPont Co. or hightemperature polyester available under the trade designation VALOX fromGeneral Electric Plastics Co. Resilient member 150 may advantageously beformed of beryllium-copper alloy of thickness between about 5 and 10mils (0.125 and 0.25 mm) to provide good conductivity when in contactwith plate 115 and reliable spring action when the pressure of pellet175 against it is removed. Additionally, the resilient arm 150 may beplated with silver or gold to increase its conductivity. The thermalpellet 175 is advantageously formed of a polymer having a relatively lowmelting point, e.g., between about 65° C. and 100° C. but yet excellentcompressive strength to keep the resilient arm 150 in place duringnormal cell operation. A suitable material for thermal pellet 175 havingsuch properties is a polyethylene wax available under the tradedesignation POLYWAX from Petrolyte Co. A thermal pellet 175 of suchpolyethylene wax melts within a desirable temperature range of betweenabout 75° C. and 80° C.

Another alternative embodiment is shown in FIGS. 9 and 10. Thisembodiment is essentially the same as the embodiment depicted in FIGS.1-2 and for the same application except that the thermally responsivecurrent interrupt mechanism comprises a shape memory alloy member 45 inplace of bimetallic disk 40 and spring 50. A shape memory alloy is analloy by which a plastically deformed metal is restored to its originalshape by a solid state phase change caused by heating. The shape memoryresponse is brought about by the strong crystallographic relationshipbetween the phase stable at low temperature known as martensite and thephase stable at high temperature known as austenite. The shape to beremembered is formed in the austenite phase when the alloy is held inplace and heated and when the alloy is cooled the material develops themartensitic structure. In use when the shape memory alloy is heated itreturns to its remembered shape in austenite phase. Shape memory alloysfor the application described herein are commercially available, forexample, from Special Metals Corp. of New Hartford , N.Y.

The shape memory alloy member 45 (FIG. 9) is electrically conductive andmay advantageously have the same shape as spring 50 (FIG. 1). It thusmay be desirably formed of a single partially hollow disk, for example,a disk 45 having a discontinuous surface, namely, a surface with anaperture 45f (FIG. 5) therethrough. The disk 45 has an outer edge 45a(FIG. 5) and a flexible portion 45b protruding inwardly into the hollowportion, e.g. into said aperture, from peripheral edge 45a. The flexibleportion 45b is preformed advantageously with a slight bend 45e in itssurface as shown in FIG. 9 so that its terminal end 45c rests flatagainst contact plate 15 to complete the electrical pathway betweenpositive electrode 88 and end cap 20. The thickness of flexible portion45b is smaller than its length. The thickness of flexible portion 45b isdesirably less than 1 mm, preferably between 0.2 and 0.5 mm. Flexiblemember 45b is oriented horizontally within end cap assembly 10 as shownin FIG. 9 so that the current path through member 45b will be at leastsubstantially in the direction of its thickness and preferablyessentially all of the current path through member 45b may be in thedirection of its thickness. If the cell internal temperature becomes toohigh for any reason, e.g., occurrence of an uncontrolled exothermicreaction, memory metal member 45 will flex and flatten thereby breakingelectrical contact with contact plate 15. In application to rechargeablebatteries, particularly lithium ion cells, the memory metal alloy 45 maybe preformed by the manufacturer to flex desirably at a temperaturebetween 60° C. and 120° C. When such temperature is reached memory metalmember 45 will immediately flex to interrupt current flow and shut downthe cell, thus preventing exothermic reaction which could lead to athermal runaway situation.

A desirable shape memory alloy to activate at such temperatures may beselected from known memory alloy groups, for example, nickel-titanium(Ni--Ti), copper-zinc-aluminum (Cu--Zn--Al), and copper-aluminum-nickel(Cu--Al--Ni). However, it has been determined that the most desirablealloy for shape memory alloy member 45 is a nickel-titanium alloy forapplication as the above described current interrupter forelectrochemical cells. A preferred nickel titanium memory alloy isavailable under the trade designation NITINOL alloy from Special MetalsCorporation. The shape memory member 45 may be of resettable alloy, thatis, one that deforms when heated but returns to its original shape uponcooling without application of external force. However, the shape memorymetal 45 for the present application need not be resettable, that is,may irreversibly deform when heated to its activation temperature. Thememory element 45 of the preferred alloy material NITINOL alloy isnormally fabricated such that it is not resettable once it is activated.The preferred memory element 45 of NITINOL alloy may conveniently befabricated as a single piece having a circular peripheral edge (FIG. 5)from which protrudes inwardly a flexible portion. The flexible portion45b may conveniently be of rectangular shape and comprising an outerportion 45d and an inner portion 45c separated by bend 45e. Memoryelement 45 flexes along bend 45e when a predetermined activationtemperature, preferably a temperature between about 60° C. and 120° C.is reached, element 45b deflects away from contact with contact plate 15as shown in FIG. 10, thus interrupting current flow within the cell. Inorder to achieve such activation effect it has been determined that thethickness of the NITINOL alloy member 45 may advantageously be in arange between about 0.2 and 0.5 mm with a surface area such that theresistance of said member is less than about 5 milli-ohm. The abovedescribed shape for memory member 45, namely a hollow disk having acircular outer edge 45a from which protrudes inwardly a flexible portion45b is desirable, since it allows for reduced thickness and good contactarea to reduce the overall resistance of member 45. The shape memorymember 45 desirably does not have a deformation strain of more thanabout 8 percent. Also, the angle between portion 45c and 45d isdesirably between about 10 and 30 degrees. This allows memory member 45bto deflect away from contact plate 15 and flatten when the activationtemperature is reached. In application to lithium ion cells the abovedescribed preferred design for memory member 45 may result in itsoverall resistance being less than 5-milliohm which in turn allows acurrent drain of up to 5 amp under continuous cell operation. Although arectangular shape for the flexible portion 45b is desirable other shapesmay also be used, for example, the flexible portion member 45 may betapered somewhat so that the average width of 45b is smaller that theaverage width of portion 45d. In the preferred design the bottom surfaceof memory element 45 may be smooth, that is without legs or protrusionsemanating therefrom.

Memory metal 45 has the advantage of requiring fewer components in thatspring 50 used in connection with the bimetallic disk embodiment can beeliminated. Thus, memory element 45 has built into a single element dualproperties of heat responsive activation and spring-like flexing uponsuch activation. Also the spring-like force per unit mass of memoryelement 45 is greater than that of bimetallic disk 40. This propertyrenders the memory element 45 particularly suitable for application tosmall cells, for example AAA size cells or thin prismatic rechargeablecells. In this case of cells having a diameter or width smaller than thewidth of AAA size cells, width less than about 10.0 mm, the rupturablepressure diaphragm 70 may be eliminated because of the small availablespace within the end cap assembly 10. In such application the memorymember 45 and above described preferred structural design for end capassembly 10 could still be employed except that support plate 60 anddiaphragm 70 could be merged into a single member having the shape andorientation of plate 60 shown in FIG. 9. In such design lower chamber 78and vent holes 63 would be eliminated. Contact plate 15 may also beoptional, since the shape memory member 45b may be in direct electricalcontact with plate 60 which could thus function as the contact plate.

A preferred embodiment of an end cap assembly incorporating a memorymetal element 45 has also a pressure responsive diaphragm 70 whichactivates to expand in the manner illustrated in FIG. 3 to break theelectrical pathway between positive electrode 88 and end cap plate 20when pressure within the cell reaches a predetermined level. Also aspreviously described with reference to the embodiments shown in FIGS.1-3 pressure diaphragm 70 (FIGS. 9 and 10) may also have grooves 63which are designed to rupture to let gas pass therethrough when pressurewithin the cell exceeds a predetermined level. If the pressure build-upwithin the cell is drastic, the central portion 45 of diaphragm 70 willalso rupture thereby allowing a wider area for escape of gas from theinterior of the cell.

An enlarged view of the end cap assembly 10 of FIG. 1 and 9 is shown inFIG. 5. The end cap assembly 10 may be made by assembling the componentsshown in FIG. 5 in the following order: With reference to constructionof the embodiment shown in FIG. 1 a preassembly is formed comprisingcomponents 20, 50, 40, 35, 15, 60, 70, 25, and 55. With reference toconstruction of the embodiment shown in FIG. 9 the preassembly is formedcomprising components 20, 45, 35, 15, 60, 70, 25, and 55.

The preassembly of the embodiment shown in FIG. 1 is convenientlyaccomplished by first inserting plastic grommet 25 into crimp ring 55,then inserting vent diaphragm 70 into grommet 25 and then insertingsupport plate 60 with contact plate 15 welded thereto into ventdiaphragm 70. Thereupon insulator disc 35 is placed around contact plate15 and bimetallic disk 40 is placed to rest on outwardly sloping arm 35bof insulator disk 35. Bimetallic disk 40 is not bonded to insulator disk35 but rests thereon in a free floating condition, with the insulatordisk helping to act as positioning means for the bimetallic disk. Thetop surface of the outer end of resilient springlike member 50 is weldedto the circumferential edge of end cap 20. The end cap 20 with resilientmember 50 welded thereto is then placed over insulator disk 35 so thatthe raised central portion of contact plate 15 contacts the interior endof resilient member 50 and the bottom surface of the outer end ofresilient member 50 contacts the circumferential edge of insulator disk35. Thus, the outer end of resilient member 50 is wedged between end cap20 and insulator disk 35 and the opposite or inner end of member 50 isin contact with contact plate 15.

The preassembly of the embodiment shown in FIG. 9 is constructed insimilar manner except that bimetallic disk 40 is eliminated and theshape memory member 45 is employed in place of spring 50. Thus, thepreassembly of the embodiment shown in FIG. 9 is convenientlyaccomplished by first inserting plastic grommet 25 into crimp ring 55,then inserting vent diaphragm 70 into grommet 25 and then insertingsupport plate 60 with contact plate 15 welded thereto into ventdiaphragm 70. Memory member 45 is then welded to the circumferentialedge of end cap 20. The end cap 20 with memory member 45 welded theretois then placed over insulator disk 35 so that the raised central portionof contact plate 15 contacts the interior end of memory member 45 andthe bottom surface of the outer end of memory member 45 contacts thecircumferential edge of insulator disk 35. Thus, the outer end of memorymember 45 is wedged between end cap 20 and insulator disk 35 and theopposite or inner end of member 45 is in contact with contact plate 15.

With reference to construction of either the embodiment of FIG. 1 or 9the ring 55 is then mechanically crimped over the top edge of grommet 25to hold the top end of grommet 25 tightly pressed against thecircumferential edge of end cap 20. This crimping is accomplished byapplying mechanical force along the centroidal (vertical) axis of ring55. Then in a second crimping step mechanical pressure is appliedradially to the walls of crimp ring 55, thereby completing assembly ofthe preassembly. The radial crimping serves to keep the preassemblyinternal components tightly and hermetically sealed within the ring 55.The preassembly is then inserted into metallic cover 30 so that thebottom surface of crimp ring 55 rests against the bottom inside edge ofcover 30. Thereupon the bottom surface of crimp ring 55 is welded to thebottom inside surface of cover 30. Pressure plate 80 is then snappedinto the bottom of insulator standoff ring 42 and the standoff ring 42with pressure plate 80 attached thereto is then placed against theoutside bottom surface of cover 30 so that the raised central portion ofpressure plate 80 contacts vent diaphragm 70. This point of contactbetween pressure plate 80 and diaphragm 70 is then spot welded thuscompleting construction of end cap assembly 10. The end cap assembly 10may be applied to a cell, for example, by inserting it into the open endof the cylindrical casing 90 of a cell as shown in FIG. 7 and weldingthe outer surface of cover 30 to the inside surface of the cylindricalcasing 90 at the open end 95 thereof. This results in cell 100 shown inFIG. 8 with end cap assembly 10 being tightly sealed within thecylindrical casing 90 and the end cap plate 20 forming a terminal of thecell.

While this invention has been described in terms of certain preferredembodiments, the invention is not to be limited to the specificembodiments but rather is defined by the claims and equivalents thereof.

What is claimed is:
 1. An end cap assembly for application to anelectrochemical cell having a positive and a negative terminal and apair of internal electrodes (anode and cathode), said end cap assemblycomprising a housing, a chamber within the housing, and an exposed endcap plate, said plate functional as a cell terminal, said end capassembly having an electrically conductive pathway therethroughpermitting the end cap plate to be electrically connected to a cellelectrode when said end cap assembly is applied to a cell, said end capassembly further comprising a) thermally responsive means for preventingcurrent from flowing through said electrical pathway, wherein saidthermally responsive means comprises a shape memory alloy memberactivatable when the temperature within said end cap assembly reaches apredetermined level causing a break in said electrical pathway, and b) apressure responsive means comprising a rupturable member located at theend of said end cap assembly opposite said end cap plate, saidrupturable member rupturing when gas pressure on the side thereoffurthest from said end cap plate reaches a predetermined level producinga rupture in said member allowing gas to pass therethrough.
 2. The endcap assembly of claim 1 wherein said shape memory alloy comprises anickel-titanium alloy.
 3. The end cap assembly of claim 1 wherein saidcell has a cylindrical casing and the end cap assembly is applied to thecell by inserting it into the open end of the cylindrical casing andwelding the end cap assembly to the casing.
 4. The end cap assembly ofclaim 1 wherein said thermally responsive means comprises a chamberwithin said end cap assembly, the shape memory alloy member forming aportion of said electrical pathway, wherein when the cell temperaturewithin said assembly reaches a predetermined level the shape memoryalloy member deforms thereby causing a break in said electrical pathway.5. The end cap assembly of claim 4 wherein said shape memory elementcomprises a flexible member of single piece construction having a bentsurface wherein when the cell temperature within said assembly reaches apredetermined level the surface of said shape memory member deformscausing a break in said electrical pathway.
 6. The end cap assembly ofclaim 4 wherein said shape memory member comprises a disk having anaperture therethrough, said disk having an outer edge with a flexibleportion protruding into said aperture from a portion of the outer edge,wherein the outer edge rests on a surface of an insulating member withinsaid end cap assembly, wherein said flexible portion has a bent surface,wherein when the cell temperature within said assembly reaches apredetermined level said bent surface deforms causing a break in saidelectrical pathway.
 7. The end cap assembly of claim 6 wherein saidouter edge of said shape memory member is sandwiched between a portionof said end cap plate and a portion of said electrically insulatingmember and wherein the end cap assembly comprises a contact plate whichforms part of said electrical pathway, wherein said shape memory memberis in electrical contact with said contact plate.
 8. The end capassembly of claim 1 further comprising a separation member placed acrossthe interior width of the end cap assembly and between said end capplate and said rupturable member, said separation member separating saidthermal responsive means from said pressure responsive means.
 9. The endcap assembly of claim 8 wherein said separation member comprises ametallic plate having at least one aperture therein so that when saidrupturable member ruptures gas passes through said aperture and intosaid chamber within said end cap assembly.
 10. The end cap assembly ofclaim 8 wherein said end cap plate has at least one aperturetherethrough so that when said rupturable member ruptures gas collectedfrom said chamber passes through said end cap aperture and to theexternal environment.
 11. The end cap assembly of claim 10 wherein saidrupturable member comprises a rupturable diaphragm.
 12. The end capassembly of claim 11 further comprising an electrically insulatinggrommet in contact with the peripheral edge of the end cap plate and theperipheral edge of the diaphragm, said end cap assembly furthercomprising a metallic member (crimping member) mechanically crimpedaround said grommet to hold said diaphragm and said end cap plate undermechanical compression.
 13. The end cap assembly of claim 12 furthercomprising a metallic cover around said crimping member.
 14. The end capassembly of claim 13 wherein the end cap assembly is applied to a cellby inserting it into the open end of a cylindrical casing for the celland welding the outside surface of said cover to the inside surface ofsaid casing, whereupon the end cap assembly becomes tightly sealedwithin the cylindrical case with the end cap plate comprising a terminalof the cell being exposed to the external environment.
 15. The end capassembly of claim 1 wherein said shape memory alloy member comprises adisk having an aperture through the thickness thereof, said shape memorymember having a peripheral edge with a flexible portion protrudinginwardly into said aperture from a portion of said peripheral edge,wherein said edge rests on a surface of an insulating member within saidend cap assembly, wherein when the cell temperature within the cellassembly reaches a predetermined level said flexible portion deformscausing a break in said electrical pathway.
 16. In an electrochemicalcell of the type formed by an end cap assembly inserted into an openended cylindrical case for the cell, said cell further having a positiveand a negative terminal and a pair of internal electrodes (anode andcathode), wherein said end cap assembly has a housing and an exposed endcap plate, said end cap plate functional as a cell terminal, theimprovement comprising said end cap plate being electrically connectedto one of said electrodes through an electrically conductive pathwaywithin said end cap assembly, wherein said end cap assembly comprises a)thermally responsive means comprising an electrically conductive shapememory alloy member for preventing current from flowing through thecell, wherein said shape memory member comprises a flexible memberhaving a bent surface and a thickness smaller than its length, saidflexible member oriented within said end cap assembly so that currentpasses substantially in a direction through the thickness of saidflexible member, wherein when the cell temperature reaches apredetermined temperature said shape memory member deflects along saidbent surface causing a break in said electrical pathway between said endcap plate and said electrode thereby preventing current from flowingthrough the cell.
 17. The electrochemical cell of claim 16 wherein saidshape memory member comprises a disk having an aperture through thethickness thereof, said disk having a peripheral outer edge with saidflexible member protruding inwardly into said aperture from a portion ofsaid peripheral edge, said flexible member having a bent surface whichdeflects when the cell temperature reaches a predetermined level. 18.The electrochemical cell of claim 16 wherein said shape memory membercomprises a nickel-titanium alloy.
 19. The electrochemical cell of claim16 further comprising b) pressure responsive means allowing gas from theinterior of the cell to pass into the interior of said end cap assemblywhen internal gas pressure within the cell reaches a predeterminedlevel.
 20. The electrochemical cell of claim 19 wherein said pressureresponsive means comprises a rupturable diaphragm plate located at theend of said end cap assembly opposite said end cap plate, said diaphragmplate rupturing when gas pressure on the side thereof furthest from saidend cap plate reaches a predetermined level producing a rupture in saiddiaphragm allowing gas to pass therethrough.
 21. The electrochemicalcell of claim 20 further comprising a separation member placed acrossthe interior width of the end cap assembly and between said end capplate and said rupturable diaphragm, said separation member separatingsaid thermal responsive means from said pressure responsive means. 22.The electrochemical cell of claim 21 wherein the end cap plate has atleast one aperture therethrough so that gas produced in the cell maypass through the end cap aperture to the external environment.
 23. Theelectrochemical cell of claim 20 wherein the end cap assembly comprisesa contact plate therein which forms part of said electrical pathway,wherein said shape memory member is in electrical contact with saidcontact plate.
 24. The electrochemical cell of claim 18 wherein when thecell temperature reaches a value between about 60° C. and 120° C. saidshape memory member deforms causing the shape memory member to sever itselectrical connection with said contact plate.
 25. The electrochemicalcell of claim 20 further comprising an electrically insulating grommetin contact with the peripheral edge of the end cap plate and theperipheral edge of the diaphragm plate, said end cap assembly furthercomprising a metallic member (crimping member) mechanically crimpedaround said grommet to hold said diaphragm plate and said end cap plateunder mechanical compression.
 26. The electrochemical cell of claim 25further comprising a metallic cover around said crimping member.
 27. Theelectrochemical cell of claim 26 wherein the end cap assembly is appliedto a cell by inserting it into the open end of a cylindrical casing forthe cell and welding the outside surface of said cover to the insidesurface of said casing, whereupon the end cap assembly becomes tightlysealed within the cylindrical case with the end cap plate comprising aterminal of the cell being exposed to the external environment.
 28. Anend cap assembly for application to an electrochemical cell having apositive and a negative terminal and a pair of internal electrodes(anode and cathode), said end cap assembly comprising a housing and anexposed end cap plate, said plate functional as a cell terminal, saidend cap assembly having an electrically conductive pathway therethroughpermitting the end cap plate to be electrically connected to a cellelectrode when said end cap assembly is applied to a cell, said end capassembly further comprising a) thermally responsive means for preventingcurrent from flowing through said electrical pathway, wherein saidthermally responsive means is activatable when the temperature withinsaid end cap assembly reaches a predetermined level causing a break insaid electrical pathway, wherein said thermally responsive meanscomprises a chamber within said end cap assembly and further comprises ashape memory alloy member, the shape memory alloy member forming aportion of said electrical pathway, wherein a portion of said shapememory alloy member comprises a disk having an aperture through thethickness thereof, said disk having an outer edge with a flexibleportion protruding inwardly into said aperture from a portion of saidouter edge, wherein the outer edge rests on a surface of an insulatingmember within said end cap assembly, wherein when the cell temperaturewithin said assembly reaches a predetermined level said flexible portiondeforms causing a break in the electrical pathway.